Field of the Invention
The present invention relates to a surveillance system for purposes of detecting the presence of unallowed objects in a predetermined restricted space.
Brief Description of the Prior Art
An ideal security system should have a zero false alarm probability as well as zero missing target probability. Striving to this goal the systems are thoroughly designed starting with sensor physics through sensor data processing to decision making logic. A particular application area of the security system applies its specific requirements to the system. For instance, if some objects potentially detectable as targets are allowed in the surveyed area, they must be recognized properly.
Often a single sensor like passive infrared detector or radar alone cannot provide enough information about alarm condition. Thus multiple sensors are used for capturing the raw data, and more processing power is needed for increased amount of data. This leads to higher complexity and hence higher cost of the security system. An obvious goal is combining the data coming from different types of sensors in the most effective way to achieve high security at reasonable cost.
Alarm conditions can be defined as presence of a living object in the restricted area. This means the system is required to distinguish between living and non-living objects basing on their properties like infrared radiation or body movement due to breathing and heartbeat. Another alarm conditions can be set as changes to some reference picture observed by sensors. In this case the system keeps a set of reference data acquired usually during its deployment time. If the sensors used in the system have high resolution, and the covered area/volume is large, then the amount of data necessary to keep and proceed greatly increases, causing higher complexity and cost of the system.
One example of security system based on more than one type of primary sensors is described in U.S. Pat. No. 8,330,647. It uses at least one radar plus long-wave infrared sensor for detection, tracking, and classification of people and vehicles automatically and in real time for border, property, and facility security surveillance. This system applies a complex algorithm for radar data by means of a supercomputer, however the data from infrared sensor are used mostly by operator for identification of the target. It operates by radar resolution cells without effectively combining radar data with infrared sensors data, or it works on infrared sensor data alone when triggered by possible target detection based on radar data. This system covers a large area, but operates only in two-dimensional plane, and therefore is not good enough for a small restricted area like elevator car.
Another example of security system based on more than one type of primary sensors is described in BG103383, where a combination of radar and video cameras is used, the information from which is summed up in order to obtain a complete and reliable picture of the situation observed in a specific area. For this system the information from different sensors is presented on monitors for an operator who eventually is responsible for making a decision based in this information.
An example of taking into account an environment for radar-based security system is U.S. Pat. No. 7,236,124, where a radar system transmits an environment-sensing pulse, and a processing circuitry time-reverses an order of radar return samples and generates a convolution matrix from the radar return samples resulting from a transmission of the environment-sensing pulse. Then the system radiates a signal generated specifically for this environment. This method effectively extends the dynamic range of the system in terms of its capability to detect small targets whose reflections would otherwise be masked off by reflections from large environment objects situated at the same distance. An obvious drawback is higher complexity since it needs a properly generated radiated signal for each environment instead of a simple pulse.
Another example of taking into account an environment for radar based security system is U.S. Pat. No. 4,197,537, where simple radar with single transmitting antenna is equipped with additional analogue or digital circuit providing its adaptation for environment. Using pre-stored reflected signal data corresponding to secure environment it is then able to detect changes in reflected signal at any distance and comparing them against the reference signal can activate alarm. Albeit simple and inexpensive, this method can't distinguish between living and non-living objects. Also it is equally sensitive to reflection changes coming from different objects situated at the same distance but at different angles. Beside this, multiple reflections produce the same effect as reflection from object at a distance corresponding to the total reflection time. As a result, its spatial resolution is not enough when it operates in restricted space like elevator car.